Expression of Stimulated by Retinoic Acid Gene 8 (Stra8) and Maturation of Murine Gonocytes and Spermatogonia Induced by Retinoic Acid In Vitro1

Abstract

Vitamin A deficiency in the mouse results in an arrest in the progression of undifferentiated spermatogonia to differentiating spermatogonia. The supplement of retinol to vitamin-A-deficient mice reinitiates spermatogenesis in a synchronous manner throughout the testes. It is unclear whether the effects of retinoids are the result of a direct action on germ cells or are indirectly mediated through Sertoli cells. The expression of Stimulated by retinoic acid gene 8 (Stra8), which is required for spermatogenesis, is directly related to the availability of retinoic acid (RA). Analysis of gene expression by microarrays revealed moderate levels of Stra8 transcript in gonocytes and high levels in A and B spermatogonia. Stra8 mRNA levels were greatly reduced or absent in germ cells once they entered meiosis. This study examined the effect of retinoic acid on cultured neonatal testes and isolated gonocytes/spermatogonia in vitro. THY1(+) and KIT(+) germ cells were isolated by magnetic-activated cell sorting from the testes of mice of different ages. Isolated germ cells were cultured and treated with either vehicle (ethanol) or RA without feeder cells. We found that 1) Stra8 is predominantly expressed in premeiotic germ cells, 2) RA stimulates gonocyte DNA replication and differentiation in cultured neonatal testes, 3) in the absence of feeder cells, RA directly induces the transition of undifferentiated spermatogonia to differentiating spermatogonia by stimulating Stra8 and Kit gene expression, 4) RA dramatically stimulates Stra8 expression in undifferentiated spermatogonia but has a lesser impact in differentiating spermatogonia, 5) endogenous Stra8 gene expression is higher in differentiating spermatogonia than in undifferentiated spermatogonia and could mediate the RA effects on spermatogonial maturation, and 6) RA stimulates a group of genes involved in the metabolism, storage, transport, and signaling of retinoids.

Stra8 and its inducer, retinoic acid, regulate meiotic initiation in both spermatogenesis and oogenesis in mice

“…Finally, in vitro studies of isolated spermatogenic cells suggest that RA acts directly on germ cells to induce Stra8 expression (9). Combined with previous reports, our present findings suggest that the meiotic initiation pathway in which Stra8 and its inducer, RA, figure so prominently is shared between the male and female germ lines in vivo.…”

“…An opportunity to explore this question experimentally arises from recent studies of Stra8 (Stimulated by retinoic acid gene 8), a vertebrate-specific gene that encodes a cytoplasmic protein and whose expression is induced by retinoic acid (6)(7)(8)(9)(10)(11). Our laboratory previously reported that, in female mice, Stra8 is expressed in embryonic ovarian germ cells shortly before they enter meiotic prophase (12).…”

ATRA and KL promote differentiation toward the meiotic program of male germ cells.

“…As previously described, 8,30 Kit and Stra8 expression were strongly upregulated in all the culture conditions in which ATRA was present ( Fig. 4A and Suppl.…”

“…It has been reported that ATRA stimulates Kit expression in isolated spermatogonia. 7,8 We confirmed these results in 4 and 7 dpn spermatogonia showing that Kit is induced at doses as low as 0.03 μM and that ATRA effect is not mediated by Sertoli cells (Suppl. Fig.…”

“…5 More recently it has been shown that the vitamin A derivative retinoic acid (RA) (either as all-trans or as 9-cis retinoic acid) promotes the expression of Stimulated by Retinoic Acid Gene 8 (Stra8), a key regulator of mammalian meiosis, 6 and Kit expression in undifferentiated spermatogonia. 7,8 RA functions inside the nucleus recognising two different classes of retinoid receptors. Both classes (RARs and RXRs) consist of three types of receptors, α, β and γ, encoded by distinct genes 9 and transduce RA signal by binding directly to retinoic acid responsive elements (RARE).…”